1. Field of the Invention
The present invention relates generally to acoustic systems that are used to attenuate noise. The invention involves using honeycomb to make nacelles and other structures that are useful in reducing the noise generated by aircraft engines or other noise sources. More particularly, the invention is directed to acoustic structures in which septum material is inserted into the cells of pre-existing honeycomb to provide dampening or attenuation of noise.
2. Description of Related Art
It is widely recognized that the best way of dealing with excess noise generated by a specific source is to treat the noise at the source. This is typically accomplished by adding acoustic damping structures (acoustic treatments) to the structure of the noise source. One particularly problematic noise source is the jet engine used on most passenger aircraft. Acoustic treatments are typically incorporated in the engine inlet, nacelle and exhaust structures. These acoustic treatments include acoustic resonators that contain relatively thin acoustic materials or grids that have millions of holes that create acoustic impedance to the sound energy generated by the engine. The basic problem that faces engineers is how to add these thin and flexible acoustic materials into the structural elements of the jet engine and surrounding nacelle to provide desired noise attenuation.
Honeycomb has been a popular material for use in aircraft and aerospace vehicles because it is relatively strong and lightweight. For acoustic applications, the goal has been to somehow incorporate the thin acoustic materials into the honeycomb structure so that the honeycomb cells are closed or covered. The closing of the cells with acoustic material creates the acoustic impedance upon which the resonator is based.
One approach to incorporating thin acoustic materials into honeycomb is referred to as the sandwich design. In this approach, the thin acoustic sheet is placed between two slices of honeycomb and bonded in place to form a single structure. This approach has advantages in that one can utilize sophisticated acoustic material designs that are woven, punched or etched to exact dimensions and the bonding process is relatively simple. However, a drawback of this design is that the strength of the structure is limited by the bond between the two honeycomb slices and the acoustic material. Also, the bonding surface between the two honeycomb slices is limited to the surface area along the edges of the honeycomb. In addition, there is a chance that some of the holes in the acoustic material may be unintentionally closed with excess adhesive during the bonding process.
A second approach uses relatively thick solid inserts that are individually bonded in place within the honeycomb cells. Once in place, the inserts are drilled or otherwise treated to form the holes that are necessary for the inserts to function as an acoustic material. This approach eliminates the need to bond two honeycomb slices together. The result is a strong structure in which the inserts are securely bonded. However, this approach also has a few drawbacks. For example, the cost and complexity of having to drill millions of holes in the solid inserts is a major drawback. In addition, the relatively thick solid inserts make the honeycomb stiff and difficult to form into non-planar structures, such as nacelles for jet engines.
Another approach involves inserting relatively light-weight septum fabric into the honeycomb cell to form a septum cap having anchoring flanges that are then glued to the honeycomb walls. The use of septum caps is described in U.S. Pat. Nos. 7,434,659; 7,510,052 and 7,854,298. This type of process requires that the septum caps be friction-locked within the cell to hold the septum caps in place prior to permanent bonding to the honeycomb wall. Friction-locking of the septum caps is an important aspect of this type of septum-insertion procedure. The septums may shift or otherwise move during handling if friction-locking is not adequate. Any shifting of the septums makes it difficult to apply adhesive uniformly to the septums during bonding. Shifting of the septums also causes uncontrolled altering of the acoustic properties. In the worst case, the septum may fall completely out of the honeycomb cell if friction locking is not adequate.